Win32 appends synchronized if only FILE_APPEND_DATA and SYNCHRONIZE are set
without any other flags. On the other hand, Unix requires at least the write
flag to be set together with O_APPEND.

src/core/ngx_open_file_cache.c

diff --git a/src/core/ngx_open_file_cache.c b/src/core/ngx_open_file_cache.c
index 2f6581b..cb70736 100644
--- a/src/core/ngx_open_file_cache.c
+++ b/src/core/ngx_open_file_cache.c
@@ -466,8 +466,8 @@
         fd = ngx_open_file(name, NGX_FILE_RDONLY, NGX_FILE_OPEN, 0);
 
     } else {
-        fd = ngx_open_file(name, NGX_FILE_RDWR|NGX_FILE_APPEND,
-                           NGX_FILE_CREATE_OR_OPEN, NGX_FILE_DEFAULT_ACCESS);
+        fd = ngx_open_file(name, NGX_FILE_APPEND, NGX_FILE_CREATE_OR_OPEN,
+                           NGX_FILE_DEFAULT_ACCESS);
     }
 
     if (fd == NGX_INVALID_FILE) {